Polyesters of tetraalkylcyclobutanediol



United States Patent 3,360,547 POLYESTERS 0F TETRAALKYL- CYCLUBUTANEDIOL Earl W. Wilson and Charles W. Hequembourg, Kingsport, Tenn, assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Feb. 23, 1965, Ser. No. 434,684 Claims. (Cl. 260-485) This application is a continuation-in-part application of copending US. Ser. No. 106,550 filed May 1, 1961, of which US. Ser. No. 361,926 of Apr. 6, 1964, now abandoned is a division.

This invention relates to lubricant compositions and more particularly to novel complex esters useful as thickeners for synthetic lubricants.

Synthetic ester lubricants are widely used today for a number of purposes for which they have advantages over mineral oil lubricants. A principal use is as lubricants for jet aircraft engines, which require a lubricant that can be used over wide ranges of temperature. The lubricant must remain fluid at very low temperatures yet must retain sufficient viscosity at the high operating temperatures of jet engines to lubricate properly. In addition to other stringent requirements, it must be thermally stable at such high temperatures and must not be excessively corrosive to metals with which it comes in contact.

Because no single type of ester appears to possess all of the desirable properties, it has been the practice to mix modifier ingredients with a synthetic ester that serves as the base oil of the synthetic lubricant. For instance, most base oils for synthetic lubricants require thickening to meet high temperature viscosity requirements. However, the thickeners known in the art are often not sufficently stable thermally or they are too corrosive to metals to meet specification requirements. In accordance with the present invention, we have developed novel complex esters that are useful as thickeners for ester base oils and are characterized by excellent thermal stability and low corrosivity.

In general the novel esters of our invention are complex esters or polyesters formed by reacting a dicarboxylic acid with a tetraalkylcyclobutanediol and with a chainterminating monohydric alcohol or monocarboxylic acid. The products have the general structure:

when a monohydric alcohol is used for terminating the polyester chain, or the general structure:

when a monocarboxylic acid is used for terminating the polyester chain.

In Formula I, R is the alkyl residue of the monohydric alcohol, R is the alkylene radical of the dicarboxylic acid, R is a lower alkyl radical and x is an integer from 1 to 10.

In Formula II, R is the alkyl residue of the monocarboxylic acid and the other symbols are as indicated for Formula I.

An essential feature of the complex esters of the invention is that they are derived from a tetraalkylcyclobutanediol of the formula:

wherein the substituents, R, are the same or different straight or branched chain lower alkyl groups, i.e. alkyl groups of from about 1 to 4 carbon atoms. Such diols are advantageously prepared by the method described in the patent to R. H. Hasek and E. U. Elam, U.S. 2,936,324.

The acids used in preparing the complex esters are aliphatic, dicarboxylic acids of the formula,

wherein R is an alkylene radical of about 4 to 10 carbon atoms. Examples of particularly suitable acids include adipic acid, azelaic acid, sebacic acid and decane- 1,10-dicarboxylic acid.

The rnonohydroxy alcohols used in the preparation of the novel esters of Formula I are of the formula, R OH, wherein R is a straight or branched chain alkyl group of from 1 to 20 carbon atoms. The alcohol preferably is a primary alcohol of 4 to 12 carbons atoms. Certain secondary and tertiary aliphatic alcohols can be used but are less desirable because they are less reactive and their products have lower thermal stability. The especially preferred alcohols are 2,2-disubstituted, aliphatic, primary alcohols of from 6 to 12 carbon atoms, of which 2,2-dimethyl butanol, 2,2,4-trimethylpentanol and 2,2-dimethyl decanol are examples. Such alcohols having a beta quaternary alkyl radical, are especially preferred because of the superior stability of the resulting complex ester product. Other suitable alcohols include methanol, ethanol, n-butanol, isobutanol, 2-ethylhexanol, Z-methylpentanol, Z-ethylbutanol, 3,5,5-trimethylhexanol, l-methyl petanol, lauryl alcohol, mixtures of C saturated, branched chain rnonohydroxy alcohols derived from the well-known 0x0 synthesis, cetyl alcohol, no-octadecanol, n-eicosanol, and the like.

The novel esters of the invention also include those of Formula II that are terminated with a monocarboxylic acid. The alcohol-terminated esters of Formula I have the advantage of being less corrosive than the acid-terminated esters of Formula 11. However, in uses for which acid corrosion is not a problem, the products of Formula II are valuable and exhibit in general the same useful properties as lubricant components that characterize the alcohol-terminated products. The monocarboxylic acids used in the preparation of the novel esters of Formula II are of the formula R COOH, wherein R is a straight or branched chain alkyl radical of from 1 to 19 carbon atoms, thus including acids of from 2 to 20 carbon atoms. Examples of suitable acids include acetic, propionic, nbutyric, isobutyric, 2-ethylhexanoic, pivalic, pelargonic, lauric, stearic and eicosanoic acids and the like. Preferably R is a straight or branched chain alkyl radical of from 3 to 11 carbon atoms, as in the alkanoic acids in the range from butyric to lauric.

The complex esters of our invention can be prepared by heating a mixture of the tetraalkylcyclobutanediol, the dicarboxylic acid and the rnonohydroxy alcohol or monocarboxylic acid in the presence of an esterification catalyst. Catalysts providing the most satisfactory results include: titanium esterification catalysts such as disclosed in Caldwell and Wellman, U.S. Patent No. 2,727,881, particularly titanium alkoxides such as tetraisopropyl titanate: tin esterification catalysts such as disclosed in Caldwell, U.S. Patent No. 2,720,507, and especialy tin akoxides' such as dibutyl tin oxide and alkyl tin compounds such as tetrabutyl tin: as well as various zinc salts, calcium salts, magnesium salts, and the like, known "in the. art as esterification catalysts. The esterification catalysts are usually employed in concentrations varying from 0.001 to 2 weight percent of the reaction mixture, although somewhat higher or lower concentrations can be used.

The reaction is carried out in an inert atmosphere at a temperature in the range of about 80-250 C., preferably at atmospheric pressure. When a titanium catalyst is employed, the temperature should be at least about 180 C. To avoid product decomposition, the temperature should not exceed about 220 C.

Approximately equal molar proportions of the dicarboxylic acid and the diol are employed in the reaction.

, The molar proportions of the chain terminator, i.e., the

monohydroxy alcohol or monocarboxylic acid, with respect to the other reactants can be varied, depending upon the desired chain length of the complex ester product. For a low molecular weight product, a molar proportion of the chain terminator approaching that of the diacid or the diol will be used. For products of longer chain length and higher molecular weight, the amount of the chain terminator will be substantially less than 1 mole per mole of diacid or diol. The preferred products of our invention have an average molecular weight of about 900 to 1800. (Number-average molecular weight as defined by Flory, Principles of Polymer Chemistry, Cornell University Press, 1953, pp. 273 et seq.) For obtaining such products, the proportion of the chain terminating alcohol or acid in the reaction mixture should be in the range of about 20 to 80 mole percent of the amount of diacid or diol employed.

Preferably an inert organic solvent such as benzene, toluene or xylene that forms an azeotrope with water is employed to aid in. distilling water from the reaction mixture. The reaction is continued until a product of the desired molecular weight is obtained, i.e., until the average chain length of the complex ester comprises from 1 to repeating units of the structure illustrated above (number-average molecular weight about 600-6000), and preferably until the number-average molecular weight of the product is in the range of 900 to 1800. Conveniently, the degree of completion of the reaction can be followed by measuring the acid number of the reaction mixture, stopping the reaction when the acid number drops to a level corresponding to that determined by previous tests as corresponding to the desired product. Upon comple tion of the reaction the product is normally treated with alkali to neutralize any free acid, washed with water until neutral, and further purified by distillation, preferably at subatmospheric pressure.

The complex esters of the invention are employed as minorcomponents of lubricating compositions having as amajor component a synthetic ester base oil. The complex esters of the invention are useful for improving the temperature-viscosity relationship of synthetic ester lubricants in general, including, for example, monohydroxy alcohol diesters of dibasic acids, as exemplified by the diesters of acids such as adipic, azelaic and sebacic with alcohols such as 2-ethylhexanol, 2,2,4-trimethylpentanol, 2-methylbutanol and the like; monobasic acid diesters of dihydroxy alcohols, as exemplified by the diesters of glycols such as diethylene glycol, triethylene glycol and 3-methyl-1,5-pentanediol with acids such as propionic, isobutyric, hexanoic, 2-ethylhexanoic and the like; as well as triesters such as the 'heptanoic and hexanoic acid vtriesters of trimethylolpropane.

The amount of complex ester added to the base oil should be sufficient to improve the viscosity-temperature relationship of the oil, that is, to form a mixture having a high-temperatureviscosity, e.g. at 210 F., higher than that of the base oil while having a low-temperature viscosity, e.g. at 40 F., not excessively higher than that of the base oil. Generally, an amount of complex ester in the range of about 5 to 40 weight percent of the base oil will provide the desired thickening result.

In addition to the ester base oil and the complex ester thickener, the lubricant compositions of the invention can include minor amounts of various additives such as rust inhibitors, oxidation inhibitors, V. I. improvers, pour point depressants, anti-foam additives and the like.

The following example illustrates the preparation of an especially preferred complex ester of the invention. This ester is prepared from azelaic acid, 2,2,4,4-tetramethylcyclobutanediol and 2,2,4-trimethylpentanol-1, and is characterized by exceptionally good heat stability and low corrosivity toward lead. The latter property is especially desirable for jet engine lubricants that come in contact with lead or Babbitt metal bearings.

Example In a three-liter flask fitted with a stirrer, thermometer, nitrogen inlet tube,.steam condenser connected to a Dean Stark moisture trap which in turn is connected to two water cooled condensers, are charged the following:

Grams Azelaic acid 1017 2,2,4,4-tetramethyl-1,3-cyclobutanediol 730 2,2,4-trimethylpentanol 257 Xylol All of the above ingredients were heated simultaneously under a nitrogen atmosphere to 90 C. and tetraisopropyl titanate (1.9 grams) was added as the catalyst. Water of esterification removed from the reaction at C.200 C. totaled 193 grams or 98.5% of theoretical. The reaction time, including 2.5 hours of vacuum stripping at 200 C. and 1-5 mm. of mercury, was 20.5 hours. The final crude product was diluted with heptane, washed with 5% NaOH solution and washed with water until neutral. The crude mixture was filtered through a diatomaceous earth and stripped of low boilers to 90 C. at 150-200 mm. The final product was vacuum stripped to 200 C. at 50-200 microns. The final acid number was 0.2 and the number-average molecular weight was 1100.

Table I below records physical properties of a complex ester of the invention, A, prepared as described in the above example. It also records comparative data for two complex esters B and C prepared from diols other than the tetraalkylcyclobutanediol used in preparing our complex esters. Specifically, the two comparative complex esters were prepared by the reaction of azelaic acid and 2,2,4-trimethylpentanol with 2-,methyl-1,3-pentanediol or with 2,2-dimethyl-1,3-propanediol.

Table II below records properties of synthetic ester lubricant compositions employing the complex esters of Table I as thickeners. In each of the three compositions the base oil is bis(2,2,4-trimethylpentyl)azelate. The table shows that the complex ester of our invention (A) prepared from 2,2,4,4-tetranrethyl-1,3-cyclobutanediol imparted good high-temperature viscosity to the lubricant composition without having the mentioned drawbacks of known thickening agents. The table further shows reresults of other important lubricant tests. The foaming properties of jet engine lubricants is significant because foaming in the lubricant reservoir system of the jet engine would lead to bearing starvation and ultimate failure. Our composition demonstrates satisfactorily low foaming characteristics. The lead corrosion test indicates whether or not the lubricant will be corrosive to the lead-indium alloy in the jet engine bearing. The panel coking test is an indication of the amount of coking that the lubricant will undergo in tubing, bearing, bearing housing and other areas within the hot spots of the engine. Table II shows that the lubricant composition of our invention containing polyester A was markedly superior to the compositions containing polyesters B" and C in the lead corrosion, panel coking and thermal stability tests.

and

CH CHs TABLE II.-PROPERTIES OF LUBRICANT COMPOSITIONS [Lube base, bis(2,2,4-trimethylpentyl) azelate] Viscosity (centlstokes) Foaming, Initial (r301, ml; collapse, Lead Test 2 mm. Weight Percent Panel 3 536 F.,

Polyester Coking Thermal 4 Sequence 1, Sequence Sequence 3, Stored 2 Stability 100 F. 210 F. 40 F. 75 F. 2, 200 F. 75 after Initial Weeks 21% A" 38. 5 7. 70 9,190 Non-Foam. /0. 2 N on-Foam 0. 3 2. 6 1. 0 16.5%8- 36. 54 7. 74 8,561 .do l0/0.2 d0 0.0 0.2 4.7 52. 27% O 37.24 7. 48 10,003 .-..-d0 25/03 .....d0 12.7 -82 23.7 6.2

1 Fed. Test Meth., Standard No. 791, Meth. 3211.1; AS'IM D892-46l. 2 Lead Corrosion Test, Fed. Test Method Standard N o. 791, Meth. 5321.

wherein R is an alkyl radical of from 1 to 20 carbon atoms, R is an alkylene radical of from 4 to 10 carbon atoms, the substituents, R, are lower alkyl radicals, R is an alkyl radical of 1 to 19 carbon atoms and x is an integer from 1 to 10.

2. A complex ester of the structure selected from the group consisting of C CH3 X 3 Fed. Test Meth., Standard N o. 791, Meth. 3462. 4 D. E. R. B. 2487, Issue 3, par. 6.7, Thermal Stability.

wherein R is an alkyl radical of from 1 to 20 carbon atoms, R is the alkylene radical of an acid selected from the group consisting of adipic, azelaic, sebacic and decane-l,l0-dicarboxylic acids, R is an alkyl radical of 1 to 19 carbon atoms and x is an integer from 1 to 10 corresponding to a complex ester of average molecular weight in the range 600 to 3000.

4. A complex ester of the structure selected from the group consisting of:

wherein R is a beta quaternary alkyl radical containing from 6 to 12 carbon atoms, R is the alkylene radical of an acid selected from the group consisting of adipic, azelaic, sebacic, and decane-l,l0-dicarboxylic acids, R is an alkyl radical of 3 to 11 carbon atoms, and x is an 7 integer from 1 to 10 corresponding to a complex ester of average molecular weight in the range 600 to 3000.

5. A complex ester according to claim 4 in which R is the 2,2,4-trimethylpentyl radical and R is the alkylene radical of azelaic acid.

References Cited UNITED STATES PATENTS 2,319,575 5/1943 Agens 260-77 Muskat 260-77 Szayna 260--77 Cashman et a1. 252-56 Matuszak 252-56 Examiners.

I. VAUGHN, Assistant Examiner. 

1. A COMPLEX ESTER OF THE STRUCTURE SELECTED FROM THE GROUP CONSISTING OF 